1. Field of the Invention
The present invention relates to methods and apparatus of reducing and/or controlling vortex-induced vibrations (xe2x80x9cVIVxe2x80x9d), current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations. In another aspect, the present invention relates to methods and use of passive apparatus for reducing and/or controlling VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations. In even another aspect, the present invention relates to methods and use of flow thru members with marine members for reducing and/or controlling VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
2. Description of the Related Art
When drilling for and/or producing hydrocarbons or the like from subterranean deposits which lie under a body of water, it is necessary to provide a marine riser system for establishing fluid communication between the water surface and the marine bottom. The principal purpose of the riser is to provide a fluid flow path between a drilling vessel and a well bore and to guide a drill string to the well bore.
A typical marine riser system normally consists of one or more fluid-conducting conduits which extend from the water surface to a structure (e.g., wellhead) on the marine bottom. For example, in drilling a submerged well, a drilling riser usually consists of a main conduit through which the drill string is lowered and through which the drilling mud is circulated from the lower end of the drill string back to the surface. In addition to the main conduit, it is conventional to provide auxiliary conduits, e.g., choke and kill lines, etc., which extend parallel to and are carried by the main conduit.
This drilling for and/or producing of hydrocarbons from offshore fields has created many unique engineering challenges. For example, in order to limit the angular deflections of the upper and lower ends of the riser pipe and to provide required resistance to lateral forces, it is common practice to use apparatus for adding axial tension to the riser pipe string. Further complexities are added when the drilling structure is a floating vessel, as the tensioning apparatus must accommodate considerable heave due to wave action. Still further, the lateral forces due to current drag require some means for resisting them whether the drilling structure is a floating vessel or a platform fixed to the subsurface level.
The magnitude of the stresses on the riser pipe are generally a function of and increase with the velocity of the water current passing the riser pipe, and the length of the riser pipe.
It is noted that even moderate velocity water currents acting on a riser can cause stresses. Such moderate or higher currents are readily encountered when drilling for offshore oil and gas in a marine inlet or near a river mouth.
Drilling in ever deeper water depths requires longer riser pipe strings which are subject to greater drag forces which must be resisted by more tension. This is believed to occur as the resistance to lateral forces due to the bending stresses in the riser decreases as the depth of the body of water increases. Accordingly, the adverse effects of drag forces against the riser caused by strong and shifting currents in these deeper waters increase and set up stresses in the riser which can lead to severe fatigue and/or failure of the riser if left unchecked.
There are generally two kinds of water current induced stresses.
The first kind of stress is caused by vortex-induced alternating forces that vibrate the riser (xe2x80x9cvortex-induced vibrationsxe2x80x9d or xe2x80x9cVIVxe2x80x9d) in a direction perpendicular to the direction of the current. When water flows past the riser, vortices are alternately shed from each side of the riser. This produces a fluctuating force on the riser transverse to the current. If the frequency of this harmonic load is near the resonant frequency of the riser, large vibrations transverse to the current can occur. These vibrations can, depending on the stiffness and the strength of the riser and the welds between the riser joint, lead to unacceptably short fatigue lives. In fact, stresses caused by high current conditions have been known to cause risers to break apart and fall to the ocean floor.
The second type of stress is caused by drag forces which push the riser in the direction of the current due to the riser""s resistance to fluid flow. The drag forces are amplified by vortex induced vibrations of the riser. A riser pipe that is vibrating due to vortex shedding will disrupt the flow of water around it more than a stationary riser. This results in more energy transfer from the current to the riser, and hence more drag.
Many methods have been developed to reduce vibrations of subsea risers.
Some of these methods to reduce vibrations caused by vortex shedding from subsea risers operate by stabilization of the wake. These methods include streamlined fairings, wake splitters and flags.
Streamlined, or teardrop-shaped fairings that swivel around a riser have been developed that almost eliminate the sheading or vortices. The major drawbacks to teardrop shaped fairings is the cost of the fairing and the time required to install such fairings. Additionally, the critically-required rotation of the fairing around the riser is challenged by long-term operation in the undersea environment. A non-rotating fairing subjected to a cross-current may result in vortex shedding that induces greater vibration than the bare riser would incur.
Wake splitters are flat plates that extend drom the back of a riser parallel to the current flow direction. These wake splitters have been found to be effective to create a symmetric vortex pattern so that each vortex xe2x80x9cseesxe2x80x9d an image created by the rigid splitter plate giving symmetry with respect to the axis in the direction of flow. Splitter plates also stabilize the separation points, decrease the wake width and reduce drag. Splitter plates suffer from most of the same detrimental effects as teardrop shaped fairings for off-axis currents. They must therefore either be rotatable or be used only where the directions of a significant current does not vary.
Flags are similar to wake splitters, but are flexible. They are not generally as effective as wake splitters, but have the advantage that they can wrap around a riser and remain somewhat effective with varying current directions without being rotatable. Flags are not commonly used in subsea applications due to the high probability of the flag wrapping itself around the riser and becoming ineffective, and because of the difficulty and expense of attaching the flag to the riser along the length of the riser.
Other of these methods to reduce vibrations caused by vortex shedding from subsea risers operate by modifying the boundary layer of the flow around the riser to prevent the correlation of vortex shedding along the length of the riser. Examples of such methods include the inclusion of helical strakes around the riser, axial rod shrouds and perforated shrouds.
Where possible, it is generally preferred to utilize strakes over fairings, wake splitters and flags.
There exists a need in the art for methods and apparatus of reducing and/or controlling VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
There is another need in the art for methods and apparatus of reducing and/or controlling VIV, current drag low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations that do not suffer from the disadvantages of the prior art.
There is even another need in the art for methods and apparatus that act passively to reduce and/or control VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
There is still another need in the art for methods and apparatus that use fluid passages passing thru marine members to reduce and/or control VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
It is an object of the present invention to provide for methods and apparatus of reducing and/or controlling VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
It is another object of the present invention to provide for methods and apparatus of reducing and/or controlling VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations that do not suffer from the disadvantages of the prior art.
It is even another object of the present invention to provide for methods and apparatus that act passively to reduce and/or control VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
It is still another object of the present invention to provide for methods and apparatus that use fluid passages passing thru marine members to reduce and/or control VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations.
According to one embodiment of the present invention there is provided a method for protecting a first member from the effects of a current flowing past the member, the current defining a fluid boundary layer comprising an upstream region and downstream region near the member. The method comprises providing a fluid passage thru the member with an inlet positioned in the upstream region, and an outlet positioned in the downstream region.
According to another embodiment of the present invention provides a method for protecting a member from the effects of a current flowing past that structure. The method uses a fluid passage, with an inlet and an outlet, which extends from the upstream region to the downstream region of the structure. The method involves orienting the member to position the inlet in the upstream region, and to position the outlet in the downstream region.
According to even another embodiment of the present invention, a method is provided for reducing low frequency oscillations of a moored marine vessel caused by waves or wind striking the vessel. The method involves a vessel which has a first side and a second side, and where the oscillations of the vessel define an oscillating current flowing past the vessel. The method involves providing a fluid passage extending from the first side to the second side with a first opening positioned on the first side and a second opening positioned on the second side, wherein either opening is suitable for receiving the current when the vessel moves toward said opening and either opening is suitable for discharging the current when the vessel moves away from said opening.
According to yet another embodiment of the present invention a method is provided for reducing low frequency oscillations of a moored marine vessel caused by waves or wind striking the vessel, wherein the vessel has a first side and a second side, wherein the oscillations of the vessel define an oscillating current flowing past the vessel, and wherein the vessel defines a fluid passage with a first opening positioned on the first side and a second opening positioned on the second side. The method comprises orienting the vessel to allow either opening to receive current when the vessel moves toward said opening and to allow either opening to discharge the current when the vessel moves away from said opening.
According to even yet another embodiment of the present invention, there is provided an apparatus having resistance to the effects of a current flowing past the member, the current defining a fluid boundary layer comprising an upstream region and downstream region near the member. The apparatus comprises a member defining a fluid passage thru the member with an inlet positioned in the upstream region, an outlet positioned in the downstream region; and a valve positioned to affect the flow of current thru the passage.